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Poor xj650 mpg

Discussion in 'XJ Technical Chat' started by GhostRider, May 25, 2015.

  1. GhostRider

    GhostRider New Member

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    Hi guys, I've had my 1981 xj650 with about 22,000 miles on it, close to a year now and its mileage is still not any where near it should be. I just recently made a highway trip and I believe I was getting about 22 mpg on it with no head wind and keeping it around 6000rpm. I also seem to be getting about the same mpg in town. There are no mods its all stock. I bought it in poor condition but a xj member did a bang up job fixing her up and he tried a few things like changing the fuel ratio but that didn't really seem to help. So I don't know where to start in getting my bike back up to speed so to say.
     
  2. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    You do have an aftermarket exhaust. Honestly I'm stumped too. Hopefully the collective will get this solved.

    Here's what was done (engine related work only. I did all of the safety realted items too):

    Valves adjusted, Coils epoxied, new plug wires spliced in and plug caps installed.
    Petcock was disassembled and cleaned. The petcock tested good and I did not want to add to his costs by doing a full rebuild (the owner is a college student).
    Carbs were cleaned, bench synched, and float levels wet-set.
    The intake boots had signs of shallow cracking, so they were cleaned and repaired with RTV.
    Idle mixtures were set to bunsen-blue using my Colortune.
    She starts right up and idles very well. She pulls hard in all gears, but there was (maybe still is?) a very slight bog at 3,000(?) RPM which I attribute to the aftermarket exhaust (but I could be wrong).
    The last thing I did was to replace a cracked fuel line, which I hoped was the reason for the poor fuel mileage (I had missed the leak earlier).
    One thing I do not know for sure is if I checked the carb jetting. I think they had the stock jets in them, but I just can't remember. Maybe I wrote that down in the notebook that I gave you.
     
  3. GhostRider

    GhostRider New Member

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    Yeah stock jets
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    what kind of mpg are you expecting to get? just asking. I see spec sheet page says 47mpg
     
    Last edited: May 26, 2015
  5. Toomanybikes

    Toomanybikes Well-Known Member

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    At 6000 rpm how fast is that ... 80-85 mph
    Not going to get great mpg at that speed
     
  6. Toomanybikes

    Toomanybikes Well-Known Member

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    In my 72 XS 650 service manual max gas mileage at 47 mph ( just above fast idle in 5th gear)
     
  7. XJ550H

    XJ550H Well-Known Member Premium Member

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    asking because you didn't have it on the list you said you bench synched, did you synch running?
     
  8. pygmy_goat_

    pygmy_goat_ Member

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    Yeah, let's consider the speed you were going. My bike is not 100% tuned perfectly, but I know when I go 80-85 on the freeway, the mileage is not great. Do you know the speeds you were driving on that interval?

    OTOH, I think 22 mpg is pretty bad. I believe I get 30-35 when doing fast freeway riding.
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    from another thread on mpg
    http://www.xjbikes.com/forums/threads/poor-fuel-mileage-on-my-1982-xj550-maxim.59700/
    "Engines are most efficient at their TORQUE PEAK rpm's, although I don't have the books in front of me, I'd guess that it's somewhere around 5500 - 6500 rpms. Riding at 35mph in 6th gear, while it can be done, will not result in the best fuel efficiency."

    tell us about your driving habits. do you rev the piss out of it to show off while parked?
    do you think your motor is running properly any funny sounds, noises? ride with a passenger?
     
  10. tabaka45

    tabaka45 Well-Known Member

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    22 mpg is awful, but that said, with the gas we have now you probably shouldn't expect much more than 40 mpg at best.

    I had a similar problem, but not quite as bad. Doing all the normal things got me up to about 35 mpg, and then I decided to concentrate on the pilot screws and reading the plugs. I took the colortune plug and closed the pilots to get a lean position and then opened them to get a blue flame. Then installed new plugs and rode the bike (about 20 miles each time) and adjusted the pilots (no more than the width of a dime) until I got the correct color plugs. I'm not getting about 42 on the highway and 39-40 in town. I found that it was much easier to read new plugs than to try to read plugs that had already been blackened--and probably a lot quicker also. I sounds like K-Moe did a thorough job but obviously something is amiss here. What do your plugs look like?
     
  11. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Actually that is a great idea Tabaka. I haven't looked at the plugs since the last tiem I worked on the bike near the end of last summer.

    Ghostrider if you can't post pics bring the bike over and we can have a look at the plugs as they are now. It could be that I still have the pilot circuits set too rich.
     
  12. tcoop

    tcoop Active Member

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    Is there any possibility that there are pin holes in the tank?
     
  13. GhostRider

    GhostRider New Member

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    I ride 70 mph always on highways. For my habits Im no show off I'm pretty conservative I just give enough gas to get going and usually change gears at about 4-6000 rpm
     
  14. GhostRider

    GhostRider New Member

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    IMG_20150526_180612614.jpg IMG_20150526_181047438.jpg IMG_20150526_180612614.jpg
     

    Attached Files:

  15. GhostRider

    GhostRider New Member

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    IMG_20150526_180535843.jpg
     
  16. GhostRider

    GhostRider New Member

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    There's all of the plugs.
     
  17. tabaka45

    tabaka45 Well-Known Member

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    a little rich for my taste--especially 2 of them--but I don't think enough to be all your problem. Seems to me that you must have a leak somewhere or else the engine has such low compression it just takes a bunch of gas to make it go. Have you checked the compression?
     
  18. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Yep. I set them too rich. I agree with Tabaka that there must be something else going on though. Did I happen to record compression readings in that notebook?
     
  19. XJ550H

    XJ550H Well-Known Member Premium Member

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    shift points from owners manual. I make the assumption that the factory suggested points are for maximum mileage
    shift points.PNG
     
  20. GhostRider

    GhostRider New Member

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    nope just shims and clearance on intake and exhaust recorded.
     
  21. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Ok then. I'm a bit busy at the moment, but we can make arrangements to get a compression test done in a couple of weeks. As easy as she starts I don't think she's low, but it is worth looking in to.
     
  22. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I don't think the plugs look too bad except the couple that are really dark. I'd maybe like a tad lighter in color, overall, but otherwise not a problem except the black one and the other that is close to it.

    I'd suggest that possibly the slide needles may be worn, but more likely than that would be that the emulsion tubes are worn.....where the needle goes down into them--that would allow excess gas to flow through at any speed.

    You should be getting close to 50 mpg even with this pump gas. I consistently got 48-53mpg with my xj650, and consistently 45-47mpg with my 700. Good grief, the japanese bikes are so good as sipping gas----my intruder 1500 with a stage 3 kit and K&N open filter STILL gets 37-41mpg on it.

    I won't go into other stuff at this point.....K-moe knows what he's doing. You're in good hands.
     
  23. tabaka45

    tabaka45 Well-Known Member

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    It seems that if the needles or tubes were bad he would get a much richer condition than he has. I think the gas is going somewhere other than through the engine. I wonder if there is any chance that some gas may be blowing out the overflow tubes.

    I wish you could tell me what you did to get 45-47 with your 700. I think mine is in really good shape and the best I can get is about 41-42 on the road and 39-40 in town. With such a small tank I would love to get 4-5 extra mpg.
     
  24. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I don't ride the 700 much over 65. Most of my riding is straight and relatively flat roads, and I'm in 5th much of the time. If I stay closer to 75-80mph, then it drops to about 37-40 very quickly.

    I don't have any tricks other than keeping the carbs clean and happy.

    I get 45-48mpg with my 900rk, too
     
  25. tabaka45

    tabaka45 Well-Known Member

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    You ride like I do and I'm in South Georgia--ain't nary hill in sight. Guess it's just a difference in bikes.
     
  26. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    .....or the upstate NY air......................
     
  27. gungrave9009

    gungrave9009 Member

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    I've got the exact same bike and I'm having the exact same problem. I get about 30 miles per gallon and when I have to use my reserve the bike runs pretty rough.
     
  28. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    You're having the exact same symptoms, but it may not be due to the same problem. Running rough on reserve is yet another issue. Start a thread and we'll put our heads together.
     

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